Process for producing electrode-formed glass substrate

ABSTRACT

To provide a process for producing an electrode-formed glass substrate, which increases the strength of a front substrate of a plasma display device. 
     Electrodes formed on a glass substrate are covered with a lead-free glass comprising, as represented by mass %, from 30 to 50% of B 2 O 3 , from 21 to 25% of SiO 2 , from 10 to 35% of ZnO, from 7 to 14% in total of K 2 O and either one or both of Li 2 O and Na 2 O, from 0 to 10% of Al 2 O 3 , from 0 to 10% of ZrO 2 , and from 0 to 5% of MgO+CaO+SrO+BaO, and when the molar fractions of Li 2 O, Na 2 O and K 2 O are represented by l, n and k, respectively, l is at most 0.025, and l+n+k is from 0.07 to 0.13.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lead-free glass for coveringelectrodes and an electrode-formed glass substrate, which are suitablefor producing a front substrate of a plasma display device (PDP), and aprocess for producing an electrode-formed glass substrate.

2. Discussion of Background

PDP is a representative large-screen full-color display device.

PDP is produced in such a manner that a front substrate to be used as adisplay surface and a rear substrate having a plurality of stripe- orwaffle-shaped barrier ribs formed thereon are sealed as faced with eachother, and discharge gas is introduced between such substrates.

The front substrate is one in which a plurality of display electrodepairs for inducing surface discharge are formed on a front glasssubstrate, and the electrode pairs are covered by transparent glassdielectrics. Electrode pairs usually consist of transparent electrodesmade of e.g. ITO, and bus electrodes to be formed on a part of thesurface of the transparent electrodes. As the bus electrodes, silverelectrodes or Cr—Cu—Cr electrodes are used.

On the rear substrate, barrier ribs and a fluorescent layer are formedin addition to the electrodes.

The glass (dielectrics) covering electrodes on the front substrate, isformed by e.g. a method of transferring a green sheet containing a glasspowder onto the electrodes, followed by firing, or applying a pastecontaining a glass powder on electrodes, followed by firing.

The glass forming a dielectric layer on the front substrate is requiredto be fired at a low temperature, to have high transparency afterfiring, and to have no coloration by silver diffused from the silverelectrodes. Further, along with the production of a large-sized plasmaTV, lately, the weight of a glass substrate has been brought up as anissue, and it has been studied to use a thinner glass substrate.However, in such a case, there is a concern such that the strength ofthe substrate may decrease. Therefore, in order to increase the strengthof a PDP front substrate, it has been proposed to reduce the expansioncoefficient of an electrode-covering layer (Non-patent Document 1).

Further, other than such a problem that the strength of the frontsubstrate may decrease, there is a problem such as warpage or breakingof the front substrate during firing the glass powder, and the followingmethod is suggested to solve such a problem. That is, with respect tothe linear expansion coefficients α_(A) and α_(B), of the glasssubstrate and the electrode-covering glass (electrode-covering layer),it is possible to prevent warpage or breaking of the front substrate bysatisfying (α_(A)−20×10⁻⁷/° C.)<α_(B)<α_(A) to bring the remainingstress of the glass substrate to be from −800 to +1,500 psi. Such anelectrode-covering glass is particularly preferably one having acomposition comprising, based on mass %, from 10 to 45% of B₂O₃, from0.5 to 20% of SiO₂, from 20 to 55% of ZnO, from 3 to 20% of K₂O, from 0to 10% of Na₂O, from 0 to 5% of CuO+Bi₂O₃+Sb₂O₃+CeO₂+MnO, and from 0 to30% of Nb₂O₃+La₂O₃+WO₃ (Patent Document 1).

Patent Document 1: JP-A-2006-221942 (such as “0013”, “0017”, or “0022”)

Non-Patent Document 1: 2007 SID INTERNATIONAL SYMPOSIUM DIGEST pp389-392

SUMMARY OF THE INVENTION

When the compositions, as represented by mass %, of a commerciallyavailable electrode-covering glass of a PDP front substrate wasanalyzed, it was found to comprise 35.5% of B₂O₃, 11.5% of SiO₂, 40% ofZnO, 9% of K₂₀, 1% of Na₂O, 2% of CaO, and 1% of Al₂O₃. Such a glass wasan electrode-covering glass which was regarded as particularly preferredin the above Patent Document 1.

A glass having the same composition as such electrode-covering glass,was used to cover one side of the entire conventional PDP glasssubstrate (which is PD 200 manufactured by Asahi Glass Company, Limited,wherein α_(A) is 83×10⁻⁷/° C., and which will be hereinafter sometimesreferred to as “a conventional glass substrate”), by firing at 570° C.and when the strength was measured, the falling ball strength H/H₀,which will be described later, was 1.3.

However, as mentioned above, the improvement of the strength of the PDPfront substrate has continuously been demanded.

Such a demand may possibly be fulfilled if the method suggested inNon-patent Document 1, is used, but on the other hand, it raises anotherproblem that the difference between the average linear expansioncoefficient α and the above α_(A), of the electrode-covering glass at atemperature of from 50 to 350° C. becomes too large, whereby the frontsubstrate may be deformed.

The present invention has an object to provide a glass for coveringelectrodes, a process for producing an electrode-formed glass substrate,an electrode-formed glass substrate wherein the electrodes on the glasssubstrate are covered with such a glass for covering electrodes and aglass ceramic composition for covering electrodes, which can increasethe strength of a PDP front substrate without decreasing α.

The present invention provides a lead-free glass for covering electrodes(the glass of the present invention) comprising, as represented by mass% based on the following oxides, from 30 to 50% of B₂O₃, from 21 to 25%of SiO₂, from 10 to 35% of ZnO, from 7 to 14% in total of K₂O and eitherone or both of Li₂O and Na₂O, from 0 to 10% of Al₂O₃, and from 0 to 10%of ZrO₂, wherein when it contains at least one component selected fromthe group consisting of MgO, CaO, SrO and BaO, the total of theircontents is at most 5%, and when the molar fractions of Li₂O, Na₂O andK₂O are represented by l, n and k, respectively, l is at most 0.025, andl+n+k is from 0.07 to 0.17.

Further, the present invention provides the glass of the presentinvention wherein the lead-free glass (the glass 1 of the presentinvention) has a B₂O₃ content of at least 43%, a ZnO content of at most23%, an Li₂O content of from 0 to 0.5%, an Na₂O content of from 2 to 5%,a K₂O content of from 4 to 10%, a total content of Li₂O, Na₂O and K₂O ofat most 12% and an Al₂O₃ content of from 0 to 5%, and l+n+k is at most0.11.

Further, the present invention provides a glass ceramic composition forcovering electrodes (the glass ceramic composition of the presentinvention) comprising, a powder of a lead-free glass and a powder of atitanium oxide, wherein the lead-free glass comprises, as represented bymass % based on the following oxides, from 30 to 50% of B₂O₃, from 21 to25% of SiO₂, from 10 to 23% of ZnO, from 9 to 19% in total of K₂O andeither one or both of Li₂O and Na₂O, from 0 to 10% of Al₂O₃, and from 0to 5% of ZrO₂ and when the lead-free glass contains at least onecomponent selected from the group consisting of MgO, CaO, SrO and BaO,the total of their contents is at most 5%, and when the molar fractionsof Li₂O, Na₂O and K₂O are represented by l, n and k, respectively, l isat most 0.025, and l+n+k is from 0.08 to 0.17.

Further, the present invention provides a process for producing anelectrode-formed glass substrate (the process for producing a glasssubstrate of the present invention) comprising forming electrodes on aglass substrate and covering the electrodes with glass, wherein theelectrodes are covered by the glass of the present invention.

Further, the present invention provides a process for producing anelectrode-formed glass substrate comprising forming electrodes on aglass substrate and covering the electrodes with glass, wherein theglass ceramic composition of the present invention, is fired to formglass to cover the electrodes. Moreover, such a process for producing anelectrode-formed glass substrate, belongs to the process for producing aglass substrate of the present invention.

Further, the present invention provides PDP (PDP of the presentinvention) comprising a front glass substrate to be used as a displaysurface, a rear glass substrate and barrier ribs to define cells,wherein transparent electrodes on the front glass substrate orelectrodes on the rear glass substrate are covered by the glass of thepresent invention.

In order to solve the above-mentioned problem, it is considerednecessary to find factors which influence H/H₀ by measuring H/H₀.However, as will be described later, H is one obtained by measuring thefalling ball strength of a glass specimen (a glass layer-coated glasssubstrate) made by coating a glass substrate with a glass paste,followed by firing, and one which tends to be influenced not only by theglass substrate or the glass for covering electrodes, but also by avehicle constitution or a firing condition of the glass paste.

Now, in order to increase the accuracy in the measurement of such H, itbecame clear that the number of the measurements, n, needed to be atleast 5. Consequently, it was difficult to employ the method of findingthe factors which influence H/H₀, by measuring H/H₀, since a tremendousamount of work was required for improvement of accuracy in measuring H.

Therefore, the present inventors have conducted a research for a methodwhich is capable of estimating H/H₀ without a measurement. As a result,they have found that the strength index S and the measured falling ballstrength H/H₀, were well matched as shown in Drawing 1, wherein thestrength index S was obtained by calculation by the following formula byinserting an elastic modulus E (unit: GPa), a fracture toughness valueKc (unit: MPa·m^(1/2)) and α (unit: 10⁻⁷/° C.) of the electrode-coveringglass, and α (unit: 10⁻⁷/° C.) of a glass substrate i.e. α₀. By carryingout the study by using such a method, namely, a method to estimate H/H₀by using the strength index S, the present invention has beenaccomplished. Further, with respect to the calculation for S, when α₀is, for example, 83×10⁻⁷/° C., α₀ in the following formula isrepresented by 83, and the same applies to E, Kc and α. Further, H/H₀ isapproximately S±0.2.

S=[13.314×Kc+0.181×(α₀−α)]² /E

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a graph showing the relation between the calculated value andthe measured value of the falling ball strength of a glass layer-coatedglass substrate.

FIG. 1 is obtained by using a conventional glass substrate as the glasssubstrate. The abscissa represents the above S, and the ordinaterepresents the above H/H₀. Further, the compositional ranges, asrepresented by mass %, of the electrode-covering glass used forpreparing Drawing 1, are from 1.2 to 40.6% of B₂O₃, from 0.4% to 33.3%of SiO₂, from 0 to 39.6% of ZnO, from 0 to 4.4% of Li₂O, from 0 to 4.9%of Na₂O, from 0 to 11.2% of K₂O, from 0 to 14.9% of Al₂O₃, from 0 to0.4% of MgO, from 0 to 14.6% of BaO, from 0 to 2.1% of TiO₂, from 0 to54.3% of Bi₂O₃ and from 0 to 86.1% of PbO.

E, Kc and α are, respectively, values of physical properties of theelectrode-covering glass itself, and they are not influenced by avehicle constitution or a firing condition of the glass paste.Therefore, in such a method to estimate H/H₀, there is no such problemas mentioned above in measuring H.

Kc is measured, for example, as follows.

Molten glass is poured into a stainless steel frame and annealed.

The annealed glass is formed into a plate-form glass, and its one sideis mirror-polished, followed by annealing (precise annealing) to removethe remaining stress, thereby to obtain a glass specimen having atypical size of 50 mm×50 mm and a thickness of 10 mm. Here, the preciseannealing is carried out in such a manner that, when the glasstransition point of the glass is represented by Tg, the glass is held atfrom Tg to (Tg+20° C.) for one hour and then cooled to room temperatureat a temperature-lowering rate of 1° C./min.

By using such a glass specimen, Kc is measured in accordance with JIS R1607-1995 “Testing methods for fracture toughness of fine ceramics 5. IFmethod” (indenter pressing method). That is, by using a Vickers hardnesstester, inside a globe box having a relative humidity of 35%, a Vickersindenter is pressed against the surface of the glass specimen for 15seconds, and the diagonal length of indentation and cracking length aremeasured by using a microscope attached to the tester. The Vickershardness (Hv) is obtained from the pressing load and the diagonallength, and Kc is calculated from the cracking length, Hv, E and thepressing load. The pressing load is, for example, from 100 g to 2 kg.

α is measured, for example, as follows.

The annealed glass is formed into a cylindrical form having a length of20 mm and a diameter of 5 mm, and the average linear expansioncoefficient α from 50 to 350° C. is measured by using quarts glass asstandard and a horizontal differential detection system thermaldilatometer TD 5010SA-N manufactured by Brucker AXS K.K.

E is measured, for example, as follows.

The annealed glass is formed into a plate-form having a thickness of 10mm, and the elastic modulus E is measured by JIS R 1602-1995 “Testingmethods for elastic modulus of fine ceramics 5.3 Ultrasonic pulsemethod”.

H/H₀ is measured as follows.

Typically, a glass substrate having a size of 100 mm×100 mm and athickness of 2.8 mm, is placed on a water-resistant polishing paperhaving a production particle size of #1500. From a height of 10 cm fromthe upper surface of the glass substrate, 22 g of a stainless steel ballis dropped. If the glass substrate does not break by the drop of thestainless steel ball, the dropping height is adjusted to be 10 mmhigher, and the stainless steel ball is dropped again. Until the glasssubstrate breaks, the dropping height is adjusted to be higher by 10 mmeach time, and the stainless steel ball is then dropped. Such a breakingtest of glass substrate is carried out for five times, and an averagevalue of the obtained breaking heights is represented by H₀.

H is an average value of breaking heights measured in the same manner asfor H₀, with respect to a glass layer-coated glass substrate having onesurface of the glass substrate covered with an electrode-covering glass.

That is, H is an average value of breaking heights obtained by carryingout the breaking test of the glass layer-coated glass substrate for fivetimes in the same manner as H₀ measurement, except that the surfacecovered with an electrode-covering glass is faced down and put on theabove water-resistant polishing paper.

The above glass layer-coated glass substrate is produced as follows.

100 g of a powder of the electrode-covering glass was kneaded with 25 gof an organic vehicle having 10 mass % of ethyl cellulose dissolved inα-terpineol or the like, to prepare a glass paste. The paste wasuniformly screen-printed on a glass substrate having a size of 100mm×100 mm, and dried at 120° C. for 10 minutes. Then, such a glasssubstrate was heated at a temperature-raising rate of 10° C. per minuteup to a temperature in a range of from (Ts−50° C.) to Ts, where Ts isthe softening point of the electrode-covering glass and maintained atthat temperature for 30 minutes to carry out firing, whereby anelectrode-covering glass layer was formed on the glass substrate, whichis regarded as a glass layer-coated glass substrate.

According to the present invention, it is possible to increase thestrength without decreasing α of the electrode-covering glass of the PDPfront substrate.

Further, according to a preferred mode of the present invention, it ispossible to obtain a glass for covering electrodes having a lowdielectric constant, and for example, it is possible to reduce powerconsumption of PDP. Further, when such a glass is used for coveringaddress electrodes of a PDP rear substrate, it is possible to suppressthe increase of the dielectric constant while increasing reflectance byincorporating a titanium oxide powder having a high dielectric constantto the address electrode-covering glass.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is suitable when α of the glass substrate i.e. α₀is from 78×10⁻⁷ to 88×10⁻⁷/° C., particularly from 80×10⁻⁷ to 86×10⁻⁷/°C.

The glass of the present invention is usually ground and classified, andused in the form of a powder.

In a case where the electrodes are to be covered by a glass paste, thepowdered glass of the present invention (hereinafter referred to as “theglass powder of the present invention”) is kneaded with a vehicle toobtain a glass paste. The glass paste is applied on a glass substrate onwhich electrodes such as transparent electrodes are formed, and fired toform a glass layer for covering the transparent electrodes.

In a case where the electrodes are to be covered by a green sheet, theglass powder of the present invention is kneaded with a resin, and thekneaded product obtained is applied on a supporting film such as apolyethylene film to obtain a green sheet. This green sheet istransferred onto electrodes formed, for example, on a glass substrate,and fired to form a glass layer for covering the electrodes.

Now, in the production of a PDP front substrate, such firing is carriedout typically at a temperature of at most 600° C. Further, the glasssubstrate having a glass layer formed in such a manner is the glasssubstrate of the present invention.

An average particle diameter (D₅₀) of the glass powder of the presentinvention is preferably at least 0.5 μm. If D₅₀ is less than 0.5 μm, itmay take a too much time for such powderization. D₅₀ is more preferablyat least 0.7 μm. Further, the above average particle diameter ispreferably at most 4 μm, more preferably at most 3 μm.

The maximum particle diameter of the glass powder of the presentinvention is preferably at most 20 μm. If the maximum particle diameterexceeds 20 μm, the surface of the glass layer becomes so uneven as todistort an image on the PDP in the use for formation of anelectrode-covering glass layer (transparent dielectric layer) of a PDPfront substrate, wherein the thickness is required to be usually at most30 μm. The maximum particle diameter is more preferably at most 10 μm.

Ts of the glass of the present invention is preferably at most 630° C.If it exceeds 630° C., it may be difficult to obtain a hightransmittance glass layer by the firing at a temperature of at most 600°C. It is more preferably at most 620° C., typically at most 615° C. or610° C.

Further, Ts is preferably at least 500° C. If Ts is lower than 500° C.,a resin component contained in a glass paste or a green sheet may not besufficiently decomposed in the firing step.

In a case where power consumption of PDP is to be lowered, the relativedielectric constant (∈) of the glass of the present invention at 1 MHz,is preferably at most 7.5, more preferably at most 7, particularlypreferably at most 6.4.

Kc of the glass of the present invention is preferably at least 0.74MPa·m^(1/2), more preferably at least 0.76 MPa·m^(1/2), particularlypreferably at least 0.78 MPa·m^(1/2). Kc is a value of a physicalproperty relating to the strength of a glass material, and it is animportant element to control the strength of an electrode-covering glasslayer. Further, it is also an important element to control the strengthof a glass substrate having such an electrode-covering glass layerformed on its surface, such as the glass substrate of the presentinvention or the front substrate of PDP of the present invention.

The breaking of the PDP front substrate is considered to happen in sucha manner that when an impact is exerted on the PDP front substrate, andthe substrate is deformed, an electrode-covering glass layer which ispartially in contact with barrier ribs formed on the rear substrate,crashes to such ribs and becomes damaged. However, since Kc of the glassof the present invention is at least, for example, 0.74 MPa·m^(1/2), itis considered that even if the electrode-covering glass layer becomesdamaged like above, it is rare that the damage reaches breaking.

E of the glass of the present invention is from 55 to 80 GPa, morepreferably at most 75 GPa. When the strength is desired to beparticularly high, E is more preferably at most 60 GPa.

The breaking of the PDP front substrate is considered to happen in theabove-mentioned manner such that the barrier ribs formed on the rearsubstrate and the electrode-covering glass layer crash to each other andbecome damaged. Moreover, it is considered that when E of theelectrode-covering glass layer at that time, is smaller, the impact bythe crashing is more absorbed, and damage will rarely be formed. Since Eof the glass of the present invention is, for example, at most 80 GPa,it is considered that the damage is rarely formed by crashing and hardlyreaches breaking.

The strength of glass material constituting an electrode-covering layer,is governed by Kc, etc., but in a case of the electrode-covering glasslayer-coated glass substrate, the strength of the electrode-coveringlayer becomes high or low depending on the stress formed by thedifference between α of the glass substrate i.e. α₀ and α of theelectrode-covering glass layer, in the step of cooling to roomtemperature after the step of firing to form the electrode-coveringglass layer. That is, when a of the electrode-covering glass layer issmaller than α₀, the compressional stress is exerted on the surface ofthe electrode-covering glass layer, whereby the strength of theelectrode-covering glass layer becomes high. When α is greater than α₀,the tensile stress is exerted, whereby the strength of theelectrode-covering glass layer becomes low.

When α₀ is from 80×10⁻⁷ to 86×10⁻⁷/° C., α of the glass of the presentinvention is preferably from 73×10⁻⁷ to 90×10⁻⁷/° C. If α of the glassof the present invention exceeds 90×10⁻⁷/° C., when the glass is usedfor covering electrodes on the glass substrate, the strength of theelectrode-covering glass layer-coated substrate, may decrease. α of theglass of the present invention is more preferably at most 85×10⁻⁷/° C.Further, if α of the glass of the present invention is less than73×10⁻⁷/° C., the stress to be formed by the difference with α of theglass substrate i.e. α₀, becomes too large, whereby the substrate may bedeformed or broken.

Typically, the glass of the present invention essentially comprises, asrepresented by mass % based on the following oxides, from 30 to 50% ofB₂O₃, from 21 to 25% of SiO₂, from 10 to 35% of ZnO, from 7 to 14% ofLi₂O+Na₂O+K₂O, from 0 to 10% of Al₂O₃, and from 0 to 10% of ZrO₂, and itcontains K₂O and at least one of Li₂O and Na₂O. When the glass of thepresent invention contains at least one component selected from thegroup consisting of MgO, CaO, SrO and BaO, the total of their contentsis at most 5%, and with respect to the above l, n and k, 1 is at most0.025, and l+n+k is from 0.07 to 0.13.

With reference to such a typical embodiment, components, etc. of theglass of the present invention will be as follows. Further, a molarfraction is one having the content by mol % divided by 100.

B₂O₃ is a component to stabilize the glass or to lower Ts, and isessential. Further, it has an effect to lower ∈. If B₂O₃ is less than30%, vitrification tends to be difficult. It is preferably at least 31%.For example, if a ZnO content is less than 20%, B₂O₃ is preferably atleast 35%. If B₂O₃ exceeds 50%, phase separation tends to take place, orchemical durability may decrease.

SiO₂ is a component to form the matrix of the glass, and is essential.If SiO₂ is less than 21%, Kc tends to be small and the strength tends todecrease. SiO₂ is typically at least 21.5%. If it exceeds 25%, Ts tendsto be high. It is typically at most 24%.

ZnO is a component to lower Ts and α, and is essential. If ZnO is lessthan 10%, α may be large. It is preferably at least 12%. If ZnO exceeds35%, the glass tends to be unstable. Further, Kc tends to be small. ZnOis preferably at most 32%. When the stability of the glass is desired tobe high, ZnO is preferably less than 20%.

Li₂O, Na₂O and K₂O are, respectively, components to facilitatevitrification or to lower Ts, and also components to increase α and tolower Kc.

At least one of Li₂O and Na₂O must be contained. If neither Li₂O norNa₂O is contained, Ts will be high, or warpage will be large.

When Li₂O is contained, its molar fraction l is at most 0.025. If itexceeds 0.025, there will be a large convex warpage on the side wherethe glass layer is not formed. It is considered that in alkali metal ionexchange between the electrode-covering glass layer and the glasssubstrate, Li ions having small radius will penetrate into the surfaceof the glass substrate, whereby the surface of the glass substrate incontact with the electrode-covering glass layer will shrink. l/(l+n+k)is preferably at most 0.2.

Typically, Li₂O is not contained.

Na₂O is preferably contained in a range of at most 7%. If it exceeds 7%,warpage may be large, or Kc may decrease. It is more preferably at most6%.

K₂O is a component to decrease warpage, and is essential.

K ions have large ionic radius, and are hard to transfer as comparedwith other alkali metal ions, so that it is considered that when K₂O iscontained, alkali metal ion exchange is made to be hard to proceed. K₂Ois preferably contained at least 2%, more preferably at least 5%.

However, if only K₂O is contained as an alkali metal component, when aglass layer is formed on one side of a glass substrate, a convex warpagewill be formed on the side where the glass layer is formed. It isconsidered that K ions having large ion radius penetrate into thesurface of the glass substrate, whereby the surface of the glasssubstrate in contact with the electrode-covering glass layer may expand.

When the total content R₂O of Li₂O, Na₂O and K₂O is less than 7%, andl+n+k is less than 0.07, Ts will be high. Typically, R₂O is at least 9%,and l+n+k is at least 0.09. If R₂O exceeds 14%, and l+n+k exceeds 0.13,α will be large. Further, Kc will decrease. Preferably R₂O is at most13%, and l+n+k is at most 0.12.

Al₂O₃ is not essential, but it may be contained in a range of at most10% to increase the glass stability or Kc, etc., and typically, it iscontained at least 1%. If it exceeds 10%, when silver electrodes arecovered, a phenomenon tends to take place, such that silver diffuses inthe electrode-covering glass and develops a color is (silvercoloration). It is preferably at most 7%. When it is desired to preventsilver coloration, Al₂O₃ is preferably less than 1%, more preferably notcontained.

Further, the molar fraction of Al₂O₃ is typically less than 0.04.

The total content of B₂O₃, SiO₂ and Al₂O₃ is preferably at least 55%. Ifit is less than 55%, Kc tends to be small. The total content is morepreferably at least 60%.

ZrO₂ is not essential, but it may be contained in a range of at most 10%to increase the chemical durability of the glass, to increase thestrength, etc.

The typical embodiment of the glass of the present invention essentiallycomprises the above components, and it is possible to further containother components within a range not to impair the object of the presentinvention. In such a case, the total content of components other thanthe above components, is preferably at most 12%, more preferably at most10%, typically at most 5%. Typical representatives of such componentsare as follows.

MgO, CaO, SrO and BaO, have an effect of stabilizing the glass orlowering α. For such a purpose, it is possible to incorporate at leastone member of the four components in a range of at most 5% in total oftheir contents. If it exceeds 5%, Kc tends to be small. It is morepreferably at most 3%. Further, the total of the respective molarfractions of the above four components is typically less than 0.05.

When BaO is contained, its content is preferably at most 1%. If itexceeds 1%, Kc tends to decrease. If Kc is desired to be larger, it ispreferred not to contain BaO.

When it is desired to suppress a phenomenon such that the binder is notremoved sufficiently at the time of firing, whereby carbon remains inthe glass after firing, and the glass is colored, three components suchas CuO, CeO₂ and CoO may be incorporated up to 3% in total of theircontents. If the total content exceeds 3%, the coloration of the glasswill conversely become remarkable. The total content is typically atmost 1.5%. When either one of such three components is contained, it istypical to contain CuO in a range of at most 1.5%.

For improvement of sintering property, etc., Bi₂O₃ may be contained upto 5%, but from a viewpoint such that Bi₂O₃ has a resource problem,etc., it is preferred not to contain Bi₂O₃.

A component such as TiO₂, ZrO₂, SnO₂ or MnO₂ is exemplified as acomponent which may be used for a purpose of adjusting α, Ts, chemicaldurability, glass stability, transmittance of a glass-covering layer,etc., and of suppressing the silver coloration phenomenon.

Further, the glass of the present invention does not contain PbO.

The glass 1 of the present invention is preferred when it isparticularly desired to increase the strength of the electrode-formedglass substrate. Now, the components, etc. of the glass 1 of the presentinvention will be described.

B₂O₃ is a component to stabilize the glass, to increase Kc of to lowerE, and is essential. If B₂O₃ is less than 43%, E tends to be large, andthe strength tends to decrease. It is preferably at least 44%. If B₂O₃exceeds 50%, phase separation tends to take place, or chemicaldurability may decrease. It is typically at most 49%.

SiO₂ is a component to form the matrix of the glass, and is essential.If SiO₂ is less than 21%, Kc tends to be small, or warpage tends to belarge. It is considered that the matrix component of the glassdecreases, and alkali metal ion exchange tends to take place between theelectrode-covering glass and the glass substrate. If it exceeds 25%, Tstends to be high.

The total content of B₂O₃ and SiO₂ is preferably at least 68%, morepreferably at least 70%.

ZnO is a component to decrease Ts and to lower α, and is essential. Itis also a component to increase E. If ZnO is less than 10%, α tends tobe large. It is preferably at least 11%. If ZnO exceeds 23%, the glasstends to be unstable. Further, α tends to be too large. When ∈ isdesired to be low, ZnO is preferably at most 18%, more preferably lessthan 15%.

Li₂O, Na₂O and K₂O are, respectively, components to facilitatevitrification or to decrease Ts, and also components to increase α, todecrease Kc and to increase E.

Among them, K₂O is a component to decrease warpage, and is essential.

K ions have large ionic radius, and are hard to transfer as comparedwith other alkali metal ions, so that it is considered that when K₂O iscontained, alkali metal ion exchange is made to be hard to proceed. K₂Ois preferably contained at least 5%.

However, if only K₂O is contained as an alkali metal component, when aglass layer is formed on one side of a glass substrate, a convex warpagewill be formed on the side where the glass layer is formed. It isconsidered that K ions having large ion radius penetrate into thesurface of the glass substrate, whereby the surface of the glasssubstrate in contact with the electrode-covering glass layer may expand.Further, K₂O is a component to increase ∈ and to increase α, whereby itscontent is preferably at most 9%.

Na₂O has a high effect to lower Ts, and is essential. If it is less than2%, such an effect will be insufficient. If it exceeds 5%, α will belarge.

Li₂O may be contained up to 0.5% when α is desired to be decreased.However, Li₂O is also a component to increase E remarkably, and fromsuch a viewpoint, it is usually preferably not contained. When Li₂O iscontained, its molar fraction l is at most 0.025. If it exceeds 0.025,there will be a large convex warpage on the side where the glass layeris not formed. It is considered that in alkali metal ion exchangebetween the electrode-covering glass layer and the glass substrate, Liions having small radius will penetrate into the surface of the glasssubstrate, whereby the surface of the glass substrate in contact withthe electrode-covering glass layer will shrink. l/(l+n+k) is preferablyat most 0.2.

When the total content R₂O of Li₂O, Na₂O and K₂O is less than 7%, andl+n+k is less than 0.07, Ts will be high. Typically, R₂O is at least 8%,and l+n+k is at least 0.08. If R₂O exceeds 12%, and l+n+k exceeds 0.11,α will be large or Kc will be small. Preferably R₂O is at most 10%, andl+n+k is at most 0.105.

Al₂O₃ may be contained to increase the glass stability or Kc, etc. If itexceeds 5%, when silver electrodes are covered, a phenomenon tends totake place, such that silver diffuses in the electrode-covering glassand develops a color (silver coloration). It is preferably at most 3%.When it is desired to prevent silver coloration, Al₂O₃ is preferablyless than 1%, more preferably not contained.

Further, the molar fraction of Al₂O₃ is typically less than 0.04.

ZrO₂ is not essential, but it may be contained in a range of at most 10%to increase the chemical durability of the glass, to increase thestrength, etc. If it exceeds 10%, crystallization tends to easily takeplace, or Ts tends to be high. Further, ∈ tends to be too large. It ispreferably at most 7%, more preferably at most 5%. When ∈ is desired tobe low, it is preferably at most 2%.

The glass 1 of the present invention essentially comprises the abovecomponents, and it is possible to further contain other componentswithin a range where the object of the present invention is notimpaired. In such a case, the total content of components other than theabove components, is preferably at most 5%, more preferably at most 4%,typically at most 3%. Typical representatives of such components are asfollows.

When it is desired to suppress a phenomenon such that the binder is notremoved sufficiently at the time of firing, whereby carbon remains inthe glass after firing, and the glass is colored, three components suchas CuO, CeO₂ and CoO may be incorporated up to 3% in total of theircontents. If the above total exceeds 3%, coloration of glass converselybecomes remarkable. It is typically at most 1.5%. When any one of suchthree components is incorporated, typically, CuO is contained in a rangeof at most 1.5%.

A component such as TiO₂, ZrO₂, SnO₂ or MnO₂ is exemplified as acomponent which may be used for a purpose of adjusting α, Ts, chemicaldurability, glass stability, transmittance of a glass-covering layer,etc., and of suppressing the silver coloration phenomenon. For the abovepurposes, typically, the above ZrO₂ is contained in a range of at most3%.

Further, the glass 1 of the present invention does not contain PbO.

The glass ceramic composition of the present invention is typically usedfor covering address electrodes of a PDP rear substrate.

The components of the glass ceramic composition of the present inventionand their contents will be described.

A powder of a lead-free glass is the main component of the glass ceramiccomposition for the electrode-covering layer, and is essential. Thetypical content is, as represented by mass %, from 90 to 99.9%.

Such a lead-free glass is the glass of the present invention, and itscomponents, as represented by mass %, will be described.

B₂O₃ is a component to stabilize the glass or to lower Ts or ∈, and isessential. If B₂O₃ is less than 30%, vitrification tends to bedifficult. It is preferably at least 32%, more preferably at least 35%.If B₂O₃ exceeds 50%, phase separation tends to take place, or chemicaldurability tends to decrease. It is preferably at most 47%, typically atmost 45%.

SiO₂ is a component to form the matrix of the glass and to lower ∈, andis essential. If SiO₂ is less than 21%, ∈ tends to be large. If itexceeds 25%, Ts tends to be high. It is preferably at most 23%.

ZnO is a component to lower Ts and α, and is essential. If it is lessthan 10%, α tends to be large. It is preferably at least 12%. If itexceeds 23%, the glass tends to be unstable, and ∈ tends to be toolarge. ZnO is preferably less than 20%. When α is desired to be low, itis preferably less than 15%.

Further, the molar fraction of ZnO is typically less than 0.20.

Li₂O, Na₂O and K₂O are, respectively, components to facilitatevitrification or to lower Ts, and also components to increase α, tolower Kc and to increase ∈.

Among them, at least one of Li₂O or Na₂O must be contained. If neitherLi₂O nor Na₂O is contained, Ts will be high, or warpage will be large.

When Li₂O is contained, its molar fraction l is at most 0.025. If itexceeds 0.025, there will be a large convex warpage on the side wherethe glass layer is not formed. It is considered that in alkali metal ionexchange between the electrode-covering glass layer and the glasssubstrate, Li ions having small radius will penetrate into the surfaceof the glass substrate, whereby the surface of the glass substrate incontact with the electrode-covering glass layer will shrink. l/(l+n+k)is preferably at most 0.2.

Na₂O is preferably contained in a range of at most 7%. If it exceeds 7%,warpage tends to be large, or Kc tends to decrease. It is morepreferably at most 6%.

K₂O is a component to decrease warpage, and is essential when silvercoloration is desired to be suppressed.

K ions have large ionic radius, and are hard to transfer as comparedwith other alkali metal ions, so that it is considered that when K₂O iscontained, alkali metal ion exchange is made to be hard to proceed. K₂Ois preferably contained at least 2%, more preferably at least 5%.

When the total content R₂O of Li₂O, Na₂O and K₂O is less than 10%, orl+n+k is less than 0.08, Ts will be high. Preferably, R₂O is at least10%. More preferably, R₂O is at least 12%, and l+n+k is at least 0.1.Typically, R₂O is at least 15%, and l+n+k is at least 0.12. If R₂Oexceeds 19%, or l+n+k exceeds 0.17, a will be large, or Kc willdecrease. Preferably R₂O is at most 17%, and l+n+k is at most 0.15.

Al₂O₃ is not essential, but it may be contained in a range of at most10% to increase the glass stability or Kc, etc. If it exceeds 10%,silver coloration tends to take place. It is preferably at most 8%. Whenit is desired to prevent the silver coloration, Al₂O₃ is preferably lessthan 3%, more preferably not contained.

ZrO₂ is not essential, but it may be contained in a range of at most 5%to increase the chemical durability of the glass, to increase thestrength, etc. If it exceeds 5%, Ts tends to be high. Further, ∈ tendsto be large. When ∈ is desired to be low, it is preferably at most 2%.

The typical embodiment of the lead-free glass to be used for the glassceramic composition of the present invention, essentially comprises theabove components, and it is possible to further contain other componentswithin a range where the object of the present invention is notimpaired. In such a case, the total content of components other than theabove components, is preferably at most 12%, more preferably at most10%, typically at most 5%. Typical representatives of such componentsare as follows.

MgO, CaO, SrO and BaO, respectively, are not essential, but they maysometimes have an effect of stabilizing the glass or reducing α. Forsuch a purpose, it is possible to incorporate at least one member of thefour components in a range of at most 5% in total of their contents. Ifit exceeds 5%, Kc tends to be small. Further, ∈ tends to be large. It ismore preferably at most 3%. Further, the total of the respective molarfractions of the above four components is typically less than 0.05.

When BaO is contained, its content is preferably at most 1%. If itexceeds 1%, Kc tends to decrease. If Kc is desired to be larger, it ispreferred not to contain BaO.

When it is desired to suppress a phenomenon such that the binder is notremoved sufficiently at the time of firing, whereby carbon remains inthe glass after firing, and the glass is colored, three components suchas CuO, CeO₂ and CoO may be incorporated up to 3% in total of theircontents. If the total content exceeds 3%, the coloration of the glasswill conversely become remarkable. The total content is typically atmost 1.5%.

When any one of such three components is contained, it is typical tocontain CuO in a range of at most 1.5%.

For improvement of sintering property, etc., Bi₂O₃ may be contained upto 5%, but from a viewpoint such that Bi₂O₃ has a resource problem,etc., it is preferred not to contain Bi₂O₃.

A component such as TiO₂, ZrO₂, SnO₂ or MnO₂ is exemplified as acomponent which may be used for a purpose of adjusting α, Ts, chemicaldurability, glass stability, transmittance of a glass-covering layer,etc., and of suppressing the silver coloration phenomenon. For the abovepurposes, it is possible to incorporate the above ZrO₂.

Further, the lead-free glass is preferred to have Ts of at most 600° C.and ∈ of at most 7.0.

A powder of titanium oxide is a component to increase the reflectance ofthe electrode-covering layer, and the typical content is, as representedby mass %, from 0.1 to 10%.

H/H₀ of a glass layer-coated glass substrate wherein a glass layer madeof the glass of the present invention is formed on one surface of theglass substrate, is preferably at least 1.5, more preferably at least1.7.

Further, the strength index S of such a glass layer-coated glasssubstrate is at least 1.5, more preferably at least 1.7.

As the glass substrate of the present invention, a PDP front substrateis typical, and in such a case, electrodes to be covered with the glassof the present invention are transparent electrodes of e.g. ITO, and buselectrodes such as silver electrodes or Cr—Cu—Cr electrodes, which areformed on parts of the surface of the transparent electrodes.

The process for producing the glass substrate of the present inventionis suitable as a process for producing the PDP front substrate or thePDP rear substrate, and in such a case, it is possible to carry out theprocess in the same manner as in a known production process, except forusing the glass of the present invention as a glass for covering theelectrodes of the front substrate or the rear substrate.

The PDP produced by the present invention may be a known PDP except thatthe glass of the present invention is used as a glass for covering thefront substrate electrodes or rear substrate electrodes such as addresselectrodes, and the production may be carried out in the same manner asin a known production process except that the glass of the presentinvention is used as a glass for covering the front substrate electrodesor the rear substrate electrodes.

EXAMPLES

Starting materials were formulated and mixed so that the compositionwould be as shown by mass % in lines from B₂O₃ to CuO of Examples 1 to 8and 12 in Table 1. Each mixture was heated to 1,250° C. and melted for60 minutes by means of a platinum crucible. Examples 1 to 8 representExamples of the present invention, and Example 12 represents aComparative Example. Further, in Table 2, each glass component is shownby mol %.

The obtained molten glass was partly poured into stainless-steel rollersto be processed into flakes. The glass flakes obtained were subjected todry grinding for 16 hours by an alumina ball mill, followed by airflowclassification, to prepare a glass powder having a D₅₀ of from 2 to 4μm.

Further, the rest of the above molten glass was poured into astainless-steel frame and annealed. The annealed glass was partlyprocessed into a cylindrical is shape with a length of 20 mm and adiameter of 5 mm, and using a quartz glass as a standard sample, α ofsuch a glass was measured under a load of 10 g by using a horizontaldifferential detection system thermal dilatometer, TD 5010SA-N,manufactured by Bruker AXS. The results are shown in Tables (unit:10⁻⁷/° C.).

Further, circular electrodes having a diameter of 38 mm were formed onboth sides of a plate-shape sample having a thickness of about 3 mmproduced by using a part of the annealed glass, and the relativedielectric constant ∈ at 1 MHz was measured by using LCR meter 4192 A,manufactured by Yokokawa Hewlett-Packard Company. The results are shownin Tables. Further, “-” in Tables mean that measurements were notcarried out.

Further, using such a glass powder as a sample, Ts (unit: ° C.) wasmeasured by means of a differential thermal analyzer (DTA).

The rest of the annealed glass was processed into a plate-shape having athickness of 10 mm, and the elastic modulus E (unit: GPa) was measuredin accordance with JIS R 1602-1995 “Testing methods for elastic modulusof fine ceramics 5.3 Ultrasonic pulse method”.

Further, one side of the above glass which was processed into aplate-shape, was mirror-polished, and in order to remove the remainingstress, the glass was held at a temperature of from 500° C. to 520° C.for one hour and then annealed. By using such a specimen, Kc (unit:MPa·m^(1/2)) was measured by the above method.

Further, 100 g of the above glass powder was kneaded with 25 g of anorganic vehicle having 10 mass % of ethyl cellulose dissolved inα-terpineol or the like, to prepare a glass paste. The paste wasuniformly screen-printed on the above conventional glass substratehaving a size of 100 mm×100 mm and a thickness of 2.8 mm, and dried at120° C. for 10 minutes. Then, such a glass substrate was heated at atemperature-raising rate of 10° C. per minute up to 570° C., andmaintained at the temperature for 30 minutes to carry out firing,whereby a glass layer was formed on the glass substrate.

By using values of E, Kc and α, which were obtained in the above manner,and using α value of α₀ of the glass substrate, the above strength indexS was calculated.

Further, H was measured with respect to such a glass layer-coated glasssubstrate, and by using separately measured H₀, H/H₀ was calculated.

The results of the above measurements or calculation are shown inTables. “-” in Tables means that measurements were not carried out.

Examples 9 to 11 in Table 1 are Examples of the present invention, butmelting as mentioned above was not carried out. S and the estimatedvalues of Ts, α, E and Kc obtained by calculation from the compositionsof such Examples, are also shown in Table 1.

TABLE 1 Ex. 1 2 3 4 5 6 7 8 9 10 11 12 B₂O₃ 32.0 31.9 40.2 40.3 43.647.8 43.8 47.0 41.2 41.1 40.7 35.5 SiO₂ 22.6 22.5 21.7 21.8 21.4 24.823.5 24.3 22.9 23.7 24.8 11.5 ZnO 30.6 30.6 17.6 17.7 13.9 16.8 17.116.5 20.6 19.5 22.0 40.0 Na₂O 4.7 4.7 4.5 4.5 5.3 4.2 4.3 3.4 4.4 3.54.4 1.0 K₂O 7.2 7.2 6.8 6.8 8.0 5.2 6.6 7.6 6.6 8.0 8.1 9.0 Al₂O₃ 2.81.4 4.4 5.9 7.3 0 0 0 4.3 4.3 0 1.0 ZrO₂ 0 1.7 3.6 1.8 0 0 3.5 0 0 0 0 0CaO 0 0 0 0 0 0 0 0 0 0 0 2.0 CuO 0 0 1.2 1.2 0.6 1.1 1.1 1.1 0 0 0 0CoO 0 0 0.1 0.1 0 0.1 0.1 0.1 0 0 0 0 Ts 602 601 599 596 587 596 598 593609 612 596 596 ε 7.4 7.4 6.6 6.6 6.5 5.9 6.6 6.0 6.4 6.4 6.6 7.9 α 8280 74 74 81 65 71 71 76 76 79 73 E 65 67 — — 55 55 56 55 58 57 62 67 Kc0.80 0.74 — — 0.82 0.83 0.78 0.81 0.81 0.82 0.76 0.65 S 1.8 1.8 — — 2.33.7 2.8 3.0 2.5 2.6 1.9 1.6 H/H₀ 1.6 1.8 2.2 2.1 — 3.9 2.7 3.1 — — — 1.3

TABLE 2 Ex. 1 2 3 4 5 6 7 8 9 10 11 12 B₂O₃ 33.0 33.0 41.6 41.6 44.747.6 44.5 47.1 42.0 42.0 41.0 37.8 SiO₂ 27.0 27.0 26.0 26.1 25.4 28.627.7 28.2 27.0 28.0 29.0 14.2 ZnO 27.0 27.0 15.6 15.6 12.2 14.3 14.914.2 18.0 17.0 19.0 36.4 Na₂O 5.5 5.5 5.2 5.2 6.1 4.7 4.9 3.8 5.0 4.05.0 1.2 K₂O 5.5 5.5 5.2 5.2 6.1 3.8 5.0 5.6 5.0 6.0 6.0 7.1 Al₂O₃ 2.01.0 3.1 4.2 5.1 0 0 0 3.0 3.0 0 0.7 ZrO₂ 0 1.0 2.1 1.1 0 0 2.0 0 0 0 0 0CaO 0 0 0 0 0 0 0 0 0 0 0 2.6 CuO 0 0 1.1 1.1 0.5 1.0 1.0 1.0 0 0 0 0CoO 0 0 0.1 0.1 0 0.1 0.1 0.1 0 0 0 0

A powder of the glass in the above Example 5 or 12, a SiO₂ powder(amorphous silica, SO—C2, manufactured by Admatechs) and a TiO₂ powder(TIPAQUE A-220, manufactured by ISHIHARA SANGYO KAISYA, LTD) were mixedso that the composition would be as shown by mass % in lines in Table 3,whereby a glass ceramic composition was prepared. Example A represents aglass ceramic composition of the present invention, and Example Brepresents Comparative Example. Further, in parentheses, the content ofeach powder is shown by volume %.

100 g of each glass ceramic composition was kneaded with 25 g of anorganic vehicle having 10 mass % of ethyl cellulose dissolved inα-terpineol or the like, to prepare a glass paste. The paste wasuniformly screen-printed on the conventional glass substrate having asize of 100 mm×100 mm and a thickness of 2.8 mm, to have a filmthickness of 20 μm after firing, and dried at 120° C. for 10 minutes.Then, such a glass substrate was heated at a temperature-raising rate of10° C. per minute up to 570° C., and maintained at the temperature for30 minutes to carry out firing.

With respect to the glass ceramic layer-coated glass substrate obtainedin such a manner, a total luminous reflectance (unit: %) at 560 nm wasmeasured by using a spectrophotometer, in accordance with JIS K 7375.The results are shown in Table 3. Further, when the substrate is usedfor a PD rear substrate, the total luminous reflectance is preferably atleast 45%.

Further, H was measured, and by using separately measured H₀, H/H₀ wascalculated. The results are shown in Table 3.

Further, measurements of dielectric constant were carried out by thefollowing method. That is, on the glass substrate, a gold paste wasapplied, followed by drying to form a lower electrode, and then, theabove glass ceramic paste was uniformly applied to have a film thicknessof 20 μm after firing, followed by drying at 120° C. for 10 minutes.Such a glass substrate was heated at a temperature-raising rate of 10°C. per minute up to 570° C., and maintained at the temperature for 30minutes to carry out firing. On the obtained film subjected to firing,the gold paste was screen-printed, followed by drying to form an upperelectrode. A dielectric constant of the fired film was measured by usingLCR meter. The results are shown in Table 3. Further, when the glassceramic composition of the present invention is used as anelectrode-covering layer of the PDP rear substrate, its dielectricconstant is preferably at most 8.5.

TABLE 3 Ex A B Type of glass 5 12 Powder of glass 90.0 (91) 95.5 (95)SiO₂ powder 5.3 (6) 1.2 (2) TiO₂ powder 4.7 (3) 3.2 (3) Dielectric 6.88.9 constant H/H₀ 2.2 1.1 Reflectance 50 50

The present invention is useful for PDP, a PDP front substrate, a PDPrear substrate, an electrode-covering glass for a PDP front substrateand an electrode-covering glass for a PDP rear substrate.

The entire disclosures of Japanese Patent Application No. 2007-184781filed on Jul. 13, 2007 and Japanese Patent Application No. 2008-127195filed on May 14, 2008 including specifications, claims, drawings andsummaries are incorporated herein by reference in their entireties.

1. A process for producing an electrode-formed glass substratecomprising forming electrodes on a glass substrate and covering theelectrodes with glass, wherein the electrodes are covered with alead-free glass comprising, as represented by mass % based on thefollowing oxides, from 30 to 50% of B₂O₃, from 21 to 25% of SiO₂, from10 to 35% of ZnO, from 7 to 14% in total of K₂O and either one or bothof Li₂O and Na₂O, from 0 to 10% of Al₂O₃ and from 0 to 10% of ZrO₂, andwhen the lead-free glass contains at least one component selected fromthe group consisting of MgO, CaO, SrO and BaO, the total of theircontents is at most 5%, and when the molar fractions of Li₂O, Na₂O andK₂O are represented by l, n and k, respectively, l is at most 0.025, andl+n+k is from 0.07 to 0.13.
 2. The process for producing anelectrode-formed glass substrate according to claim 1, wherein the abovelead-free glass has a B₂O₃ content of at least 43%, a ZnO content of atmost 23%, an Li₂O content of from 0 to 0.5%, an Na₂O content of from 2to 5%, a K₂O content of from 4 to 10%, a total content of Li₂O, Na₂O andK₂O of at most 12%, and an Al₂O₃ content of from 0 to 5%, and l+n+k isat most 0.11.
 3. The process for producing an electrode-formed glasssubstrate according to claim 1, wherein the above lead-free glass has atotal content of B₂O₃ and SiO₂ of at least 68%.
 4. The process forproducing an electrode-formed glass substrate according to claim 1,wherein the above lead-free glass does not contain Li₂O.
 5. A glassceramic composition for covering electrodes comprising a powder of alead-free glass and a powder of titanium oxide, wherein the lead-freeglass comprises, as represented by mass % based on the following oxides,from 30 to 50% of B₂O₃, from 21 to 25% of SiO₂, from 10 to 23% of ZnO,from 9 to 19% in total of K₂O and either one or both of Li₂O and Na₂O,from 0 to 10% of Al₂O₃, and from 0 to 5% of ZrO₂ and when the lead-freeglass contains at least one component selected from the group consistingof MgO, CaO, SrO and BaO, the total of their contents is at most 5%, andwhen the molar fractions of Li₂O, Na₂O and K₂O are represented by l, nand k, respectively, l is at most 0.025, and l+n+k is from 0.08 to 0.17.6. The glass ceramic composition for covering electrodes according toclaim 5, which comprises, as represented by mass %, from 90 to 99.9% ofthe powder of the above lead-free glass and from 0.1 to 10% of thepowder of titanium oxide.
 7. A process for producing an electrode-formedglass substrate comprising forming electrodes on a glass substrate andcovering the electrodes with glass, wherein the glass ceramiccomposition for covering electrodes as defined in claim 5, is fired toform glass to cover the electrodes.
 8. A process for producing anelectrode-formed glass substrate comprising forming electrodes on aglass substrate and covering the electrodes with glass, wherein theglass ceramic composition for covering electrodes as defined in claim 6,is fired to form glass to cover the electrodes.